Multiple-quantum Magic-angle Spinning Spectra Research Articles

Pushing the limit of MQMAS for low-γ quadrupolar nuclei in pharmaceutical hydrochlorides

Solid-state NMR of quadrupolar nuclei such as 35Cl has become a useful tool to characterize polymorphism in pharmaceutical hydrochlorides. The two-dimensional multiple-quantum magic-angle spinning (MQMAS) experiment can achieve isotropic resolution, and separate quadrupolar line shapes for samples with multiple sites but the pulse sequence efficiency is often low, limiting applications due to the intrinsically low NMR signals and rf field from the low gyromagnetic ratios γ. The use of cosine low-power MQMAS pulse sequences and high magnetic fields is presented to push the limit of MQMAS for insensitive low-γ quadrupolar nuclei. The improved efficiency and fields up to 35.2 T enable the acquisition of MQMAS spectra for pharmaceutical samples with multiple 35Cl sites, large quadrupolar couplings and/or in diluted dosage forms.

Separation of quadrupolar and paramagnetic shift interactions with TOP-STMAS/MQMAS in solid-state lighting phosphors.

A new approach for processing satellite-transition magic-angle spinning (STMAS) and multiple-quantum magic-angle spinning (MQMAS) data, based on the two-dimensional one-pulse (TOP) method, which separates the second-rank quadrupolar anisotropy and paramagnetic shift interactions via a double shearing transformation, is described. This method is particularly relevant in paramagnetic systems, where substantial inhomogeneous broadening may broaden the lineshapes. Furthermore, it possesses an advantage over the conventional processing of MQMAS and STMAS spectra because it overcomes the limitation on the spectral width in the indirect dimension imposed by rotor synchronization of the sampling interval. This method was applied experimentally to the Al solid-state nuclear magnetic resonance of a series of yttrium aluminum garnets (YAGs) doped with different lanthanide ions, from which the quadrupolar parameters of paramagnetically shifted and bulk unshifted sites were extracted. These parameters were then compared with density functional theory calculations, which permitted a better understanding of the local structure of Ln substituent ions in the YAG lattice.

EASY-GOING deconvolution: Automated MQMAS NMR spectrum analysis based on a model with analytical crystallite excitation efficiencies

The EASY-GOING deconvolution (EGdeconv) program is extended to enable fast and automated fitting of multiple quantum magic angle spinning (MQMAS) spectra guided by evolutionary algorithms. We implemented an analytical crystallite excitation model for spectrum simulation. Currently these efficiencies are limited to two-pulse and z-filtered 3QMAS spectra of spin 3/2 and 5/2 nuclei, whereas for higher spin-quantum numbers ideal excitation is assumed. The analytical expressions are explained in full to avoid ambiguity and facilitate others to use them. The EGdeconv program can fit interaction parameter distributions. It currently includes a Gaussian distribution for the chemical shift and an (extended) Czjzek distribution for the quadrupolar interaction. We provide three case studies to illustrate EGdeconv’s capabilities for fitting MQMAS spectra. The EGdeconv program is available as is on our website http://egdeconv.science.ru.nl for 64-bit Linux operating systems.

Q-shear transformation for MQMAS and STMAS NMR spectra

The multiple-quantum magic-angle spinning (MQMAS) and satellite-transition magic-angle spinning (STMAS) experiments refocus second-order quadrupolar broadening of half-integer quadrupolar spins in the form of two-dimensional experiments. Isotropic shearing is usually applied along the indirect dimension of the 2D spectra such that an isotropic projection free of anisotropic quadrupolar broadening can be obtained. An alternative shear transformation by a factor equal to the coherence level (quantum number) selected during the evolution period is proposed. Such a transformation eliminates chemical shift along the indirect dimension leaving only the second-order quadrupolar-induced shift and anisotropic broadening, and is expected to be particularly useful for disordered systems. This transformation, dubbed Q-shearing, can help avoid aliasing problems due to large chemical shift ranges and spinning sidebands. It can also be used as an intermediate step to the isotropic representation for expanding the spectral window of rotor-synchronized experiments.

Enhancing MQMAS of low-γ nuclei by using a high B1 field balanced probe circuit

A balanced probe circuit is used to generate high B1 magnetic field for sensitivity enhancement of multiple-quantum magic-angle spinning (MQMAS) experiment applied to low-γ quadrupolar nuclei. Electrical balancing of the sample coil can cut the peak voltage by a half, therefore improving the power handling when generating a two-fold higher B1 field. Experimental results, illustrated here with 25Mg data for two layered double hydroxides, show that the MQMAS efficiency increases more than linearly with the B1 field strength. The multiplicative enhancements from high B0 and B1 fields and an optimized MQMAS pulse sequence provide the critically needed sensitivity for acquiring MQMAS spectra of low-γ quadrupolar nuclei such as 25Mg at natural abundance.

Solid-State 17O NMR Spectroscopy of Hydrous Magnesium Silicates: Evidence for Proton Dynamics

First-principles calculations of O-17 quadrupolar and chemical shift NMR parameters were performed using CASTEP, a density functional theory (I)FT) code, to try and interpret high-resolution O-17 NMR spectra of the humite group minerals hydroxyl-chondrodite (2Mg(2)SiO(4)center dot Mg(OH)(2)) and hydroxyl-clinohumite (4Mg(2)SiO(4)center dot Mg(OH)(2)), which are models for the incorporation of water within the Earth's upper mantle. The structures of these humite minerals contain two possible crystallographically inequivalent H sites with 50% occupancy. Isotropic O-17 multiple-quantum magic angle spinning (MQMAS) spectra were therefore simulated using the calculated O-17 NMR parameters and assuming either a static or dynamic model for the positional disorder of the H atoms. Only the dynamic disorder model provided simulated spectra that agree with experimental O-17 MQMAS spectra of hydroxyl-chondrodite and hydroxyl-clinohumite. Previously published O-17 satellite-transition magic angle spinning (STMAS) spectra of these minerals showed significant dynamic line-broadenings in the isotropic frequency dimension. We were able to reproduce these line-broadenings with at least qualitative accuracy using a combination of the same dynamic model for the positional H disorder, calculated values for the change in O-17 quadrupolar NMR parameters upon H exchange, and a simple analytical model for dynamic line-broadening in MAS NMR experiments. Overall, this study shows that a combination of state-of-the-art NMR methodology and first-principles calculations of NMR parameters is able to provide information on dynamic processes in solids with atomic-scale resolution that is unobtainable by any other method.

Application of shaped adiabatic pulses to MQMAS NMR spectroscopy of spin 3/2 nuclei

Competition between nutation (r.f. driven) and adiabatic (rotor-driven) multi-quantum coherence transfer mechanisms in spin 3/2 systems results in diminished performance of rotation induced adiabatic coherence transfer (RIACT) in isotropic multiple-quantum magic-angle spinning (MQMAS) experiments for small e 2 qQ / h (<2 MHz) and high radio-frequency powers. We present a simple shaped RIACT pulse consisting of a truncated sine wave (spanning 0–0.8 π) that corrects the sensitivity losses, phase twist and relative intensity errors that can arise in MQMAS spectra utilizing constant-amplitude RIACT pulses. The shaped RIACT pulse may enhance the study of metals in biomolecules where quadrupole couplings of S = 3/2 nuclei such as 23Na tend to be small.

Total understanding of the local structures of an amorphous slag: Perspective from multi-nuclear ( 29Si, 27Al, 17O, 25Mg, and 43Ca) solid-state NMR

Local environments of all constituent elements (Si, Al, O, Mg, and Ca) in an amorphous slag were examined using multi-nuclear solid-state NMR under high magnetic field (16.4 T). The amorphous slag framework structure can be generally described as a depolymerized, chain-like network of SiO 4 tetrahedra branched with AlO 4 tetrahedra. 17O multiple-quantum magic angle spinning (MQMAS) spectrum demonstrated that oxygens occupy structurally inequivalent sites, depending on their bonding nature. 25Mg and 43Ca MQMAS spectra also showed multi-site occupancy of the ions, and the average coordination numbers were estimated to about 6 and 7, respectively. Our results should underscore the impact of the high magnetic field NMR techniques, especially MQMAS, when applied to non-crystalline solids.

Fullerene as a Standard Sample for Adjusting the NMR Spectral Resolution in Multiple-Quantum Magic-Angle-Spinning Experiments

A method using fullerene for adjusting the NMR spectral resolution for multiple quantum magic angle spinning (MQMAS) experiments is proposed. To observe its (13)C MAS signal, it is not necessary to apply (1)H decoupling, unavailable with single-resonance MQMAS probes. Although (13)C T(1) of fullerene is rather long, a recycle time of 5 s in shimming yields its signal with sufficiently high sensitivity if setting the appropriate Ernst angles corresponding to magnetic fields. It is demonstrated that so-adjusted high resolution is reflected in the (87)Rb MQMAS spectra of RbNO(3).

Multiple-quantum magic-angle spinning spectroscopy using nonlinear sampling

NMR spectroscopy is a relatively insensitive technique and many biomolecular applications operate near the limits of sensitivity and resolution. A particularly challenging example is detection of the quadrupolar nucleus 17 O , due to its low natural abundance, large quadrupole couplings, and low gyromagnetic ratio. Yet the chemical shift of 17 O spans almost 1000 ppm in organic molecules and it serves as a potentially unique reporter of hydrogen bonding in peptides, nucleic acids, and water, and as a valuable complement to 13 C and 15 N NMR. Recent developments including the multiple-quantum magic-angle spinning (MQMAS) experiment have enabled the detection of 17 O in biological solids, but very long data acquisitions are required to achieve sufficient sensitivity and resolution. Here, we perform nonlinear sampling in the indirect dimension of MQMAS experiments to substantially reduce the total acquisition time and improve sensitivity and resolution. Nonlinear sampling prevents the use of the discrete Fourier transform; instead, we employ maximum entropy (MaxEnt) reconstruction. Nonlinearly sampled MQMAS spectra are shown to provide high resolution and sensitivity in several systems, including lithium sulfate monohydrate ( LiSO 4 -H 2 17 O) and l-asparagine monohydrate ( H 2 17 O) . The combination of nonlinear sampling and MaxEnt reconstruction promises to make the application of 17 O MQMAS practical in a wider range of biological systems.

Two-Dimensional 17O Multiple Quantum Magic-Angle Spinning NMR of Organic Solids

We report two-dimensional (2D) (17)O multiple-quantum magic-angle spinning (MQMAS) NMR spectra for four (17)O-labeled organic compounds: [(17)O(2)]-D-alanine (1), potassium hydrogen [(17)O(4)]dibenzoate (2), [(17)O(4)]-D,L-glutamic acid.HCl (3) and [2,4-(17)O(2)]uracil (4). The high spectral resolution observed in the 2D (17)O MQMAS NMR spectra allows extraction of precise (17)O NMR parameters for all crystallographically distinct oxygen sites. We demonstrate that rotor synchronization is important in obtaining high-quality (17)O MQMAS spectra for organic compounds. Several issues related to the potential of (17)O MQMAS NMR for large biomolecular systems are also discussed.

Niobium-93 MQMAS NMR Spectroscopic Study of Alkali and Lead Niobates

93Nb (I=9/2) multiple-quantum magic-angle spinning (MQMAS) NMR spectra of a series of inorganic niobates have been measured. 93Nb MQMAS spectroscopy yields spectra with typically an order of magnitude higher resolution than that obtainable with 93Nb MAS spectroscopy and 93Nb dynamic-angle spinning (DAS) spectroscopy. For example, the full-width at half-maximums of the 93Nb resonances of LiNbO3 were 9 (MAS), 5.8 (DAS), and 0.7 kHz (MQMAS). Broadening of the 93Nb MAS and DAS spectra is due to the second-order quadrupolar and homonuclear dipolar interactions, respectively. The quadrupolar products (PQ) and isotropic chemical shifts (δiso) of the 93Nb resonances of LiNbO3, NaNbO3, PbNb2O6, Pb2Nb2O7, Pb3Nb2O8, Pb5Nb4O15, Pb3Nb4O13, and Pb1.83Nb1.71Mg0.29O6.39 were determined from MQMAS spectra and range from 13.6 to 26.8 MHz and from −951 to −1113 ppm, respectively. Resonances with relatively large quadrupolar coupling constants (>30 MHz) were not observed using MQMAS spectroscopy, but were detected using nutation spectroscopy. The applicability and limitations of MQMAS spectroscopy in studying inorganic niobates containing multiple 93Nb resonances are addressed and compared with those of MAS, nutation, and DAS spectroscopies.

High-temperature phase transitions ofLiRbSO4studied by magic angle spinning and multiple quantum magic angle spinning NMR of87Rb

Solid-state nuclear magnetic resonance (NMR) of the {sup 87}Rb has been employed to investigate the phase transitions in polycrystalline LiRbSO{sub 4}. The spin-lattice relaxation time T{sub 1} and the NMR line shape were measured using magic angle spinning (MAS) and multiple quantum magic angle spinning (MQMAS) methods between 373 and 488 K. The quadrupole coupling constant, Q{sub cc}, the asymmetry parameter, {eta}{sub Q}, and the isotropic chemical shift, {delta}{sub CS}, were determined from the analysis of MAS and MQMAS spectra. The anomalies of Q{sub cc} near the IV-III phase transition were associated with the deformation of the local environment of the Rb ions in the crystal lattice. The measurements of the spin-lattice relaxation rate as a function of temperature and Larmor frequency showed the existence of a damped soft mode near the V-IV and IV-III phase transitions.

Influence of glass chemical composition on the Na–O bond distance: a 23Na 3Q-MAS NMR and molecular dynamics study

The sodium environment in oxide glasses was investigated by 23Na multiple-quantum magic-angle spinning (MQ-MAS) NMR spectroscopy and compared with molecular dynamics simulations. In the experimental approach, a spectrum-inversion was employed taking into account the transfer efficiency involved in the MQ-MAS experiment. This allowed the reconstruction of the underlying two-dimensional distribution of the isotropic chemical shift correlated with the quadrupolar interaction. The isotropic chemical shift distributions were extracted from the MQ-MAS spectra to infer Na–O distance distributions. First, a Na 2O–2SiO 2 glass and its crystal analogue were characterized by this method to observe the disorder effect in the glass through the Na–O distance distribution. Thereafter, in order to study the influence of the chemical composition on the Na–O distance and distribution, additional glasses were investigated with NMR and simulation: Na 2O–5SiO 2, Na 2O–2CaO–3SiO 2, Na 2O–Al 2O 3–3SiO 2 and Na 2O–B 2O 3–3SiO 2. The molecular dynamics results are in good agreement with the experimental findings. The mean Na–O distance is higher when network formers are added to the sodium silicate glass. The effects on the Na–O distance distribution are also discussed. The simulation relates these results to the existence of several types of Na: near the non-bridging oxygen of the silicon, or as aluminum or boron charge compensator. This can be explained through charge and geometric effects.

Multiple-quantum MAS NMR of quadrupolar nuclei. Do five-, seven- and nine-quantum experiments yield higher resolution than the three-quantum experiment?

The question of whether or not higher-order (five-, seven- and nine-quantum) multiple-quantum magic angle spinning (MQMAS) experiments yield isotropic NMR spectra of half-integer quadrupolar nuclei with higher resolution than the basic three-quantum MAS experiment is examined. The frequency dispersion is shown theoretically to be greatly increased in higher-order MQMAS spectra, but it is argued that whether or not this translates into an increase in resolution depends upon the ratio of the homogeneous to inhomogeneous contributions to the isotropic linewidth. Experimentally, it is demonstrated using three-, five- and seven-quantum 45Sc MAS NMR and three- and five-quantum 27Al MAS NMR of crystalline samples that higher-order MQMAS experiments can yield a real and useful increase in resolution but that, owing to the presence of inhomogeneous broadening in the isotropic spectra, this increase is less than the theoretically predicted value. A number of practical issues relating to resolution in MQMAS NMR are also pointed out.

Strategies for extracting NMR parameters from [formula omitted] MAS, DOR and MQMAS spectra. A case study for Na 4P 2O 7

The 23 Na magic-angle spinning (MAS), double rotation (DOR) and multiple-quantum magic-angle spinning (MQMAS) NMR spectra of anhydrous sodium pyrophosphate, Na 4P 2O 7, measured at five different Larmor frequencies ( ν L) ranging from 105.8 MHz (corresponding to 400 MHz 1 H frequency) to 211.6 MHz (800 MHz) are analysed and the complete set of NMR parameters ( C qcc, η Q and δ iso) of the four crystallographically inequivalent sodium sites were determined with high accuracy. Different approaches of spectra evaluation are discussed and their results are compared. The most reliable results are obtained from a combined evaluation of five DOR and three MQMAS spectra but also from two DOR and one MAS spectra or even from a single MQMAS spectrum all data can be derived. It is shown that Na 4P 2O 7 may serve as a useful reference material for experimental set-up and reliability tests of the various NMR experiments.

Applications of27Al multiple quantum magic angle spinning NMR to aluminophosphate molecular sieves

The advantages and drawbacks of various techniques that utilize multiple quantum magic angle spinning (MQMAS) NMR for studying molecular sieves and related materials are reviewed. The techniques include z-filtered MQMAS with rotor synchronization, MQMAS with cross-polarization and MQMAS with REDOR. Several methods of recovering quantitative intensities in MQMAS spectra are discussed. Examples of applications include studies of various types of aluminophosphate. Copyright © 1999 John Wiley & Sons, Ltd.

Determination of structural data from multiple-quantum magic-angle spinning NMR experiments

It has recently been shown that multiple-quantum magic-angle spinning (MQMAS) NMR spectroscopy produces high-resolution spectra for half-integer quadrupolar nuclei which suffer second-order quadrupole coupling. The present work shows how the effects of magnified dipole-dipole coupling in MQMAS spectra can be analysed to produce quantitative structural data.

High-resolution oxygen-17 NMR spectroscopy of solids by multiple-quantum magic-angle-spinning

High-resolution solid state 17O NMR spectra of 17O-enriched compounds were observed with multiple-quantum magic-angle-spinning (MQMAS) experiments. The resolution of the 17O MQMAS spectra is approximately 30- to 150-fold higher than that found in conventional 17O MAS spectra, making it possible to detect crystallographically distinct oxygen sites. It is shown that 17O MQMAS studies are feasible for systems with 17O quadrupole coupling constants up to 7 MHz, provided that sufficiently high radio-frequency field strengths (> 120 kHz and spinning speeds (∼ 20 kHz) can be achieved.

Multiple-quantum magic-angle spinning and dynamic-angle spinning NMR spectroscopy of quadrupolar nuclei

Several aspects of the Multiple-Quantum Magic-Angle Spinning (MQMAS) technique (L. Frydman and J.S. Harwood, J. Am. Chem. Soc., 117 (1995) 5367) are compared with Dynamic-Angle Spinning (DAS). Examples of MQMAS spectra are shown for I = 3 2 nuclei with C Q up to 3.6 MHz, and for 27Al ( I = 5 2 ) with C Q up to 10 MHz. The MQMAS linewidth is largely independent of the magnitude of the homonuclear dipolar interaction, while the spinning sideband manifold is similar to that observed in DAS experiments. MQMAS is technically simple and routinely useful for studying nuclei with short spin-lattice relaxation times, but care must be taken in its use for quantitative studies as the excitation of the triple-quantum coherence is not uniform. In this regard, MQMAS is most useful for samples with small quadrupolar coupling constants. In the specific case of 17O, DAS would give spectra with excellent resolution in comparison to MQMAS. The different advantages of DAS and MQMAS make them useful complementary techniques in many cases. Two additional methods are also presented for extracting the chemical shift anisotropy (CSA) directly for quadrupolar nuclei using the multiple-quantum scheme.